The patch test (epicutaneous test) and photo patch test are used to clarify the etiology and diagnosis of allergic as well as light-potentiated contact der­ matitis. The patch test must be correlated with historic data and physical ex­ amination. Contact dermatitis constitutes a significant proportion (5%-15%) of the dis­ eases at dermatologic clinics. If the results are to be reliable, patch tests should be performed properly and the technique should not be varied in an arbitrary manner. The examiner should adhere strictly to the standard procedure so as to understand its possibilities and its limitations. Interpretation of the actual patch test reactions places high demands on the experience and skill of the examiner and on his knowledge of chemistry and pharmacology. This monograph is concerned only with the patch test method used to demonstrate the existence of contact allergy and light-potentiated contact allergy. It does not describe the "prophetic patch test" ("predictive patch test"), which is used to demonstrate the sensitizing capacity of a substance. The terminology in the text is that recommended by the International Con­ tact Dermatitis Research Group. (In this book dermatitis = eczema.) 1.2 Historic Development Joseph Jadassohn devised the epicutaneous test. In 1895 he tested a syphilitic patient who developed a cutaneous eruption after treatment with mercury ointment.